Before you even give a presentation, there are a few things
you have to do:

Choose a topic.

Submit your topic to a show to LUG. (and get it accepted!)

Create presentation from the topic.

Give the presentation.

Contrary to poular belief, each of these may be as hard as all the
others. Choosing a topic for some may be easy, based on their
expertise, whereas a Linux jack-of-all-trades may be stuck choosing
a topic. Those comfortable with public speaking may have trouble
designing the slides, and so on.